#include <stdio.h>
#include <math.h>
int main()
{
	float d,p,r,m;
	scanf("%f,%f,%f",&d,&p,&r);
	m=(log(p)-log(p-d*r))/log(1+r);
	printf("需要%5.1f月还清\n",m);
	return 0;
}

